First off, this thread is not meant to be completely encompassing. I simply took a machine that I had on hand and I tested the installation time, the disk space consumed, and the bootup and shut down time of both of the operating systems.
My test box
Dell Optiplex 755
Core 2 Duo E7200 @ 2.53Ghz
160GB Seagate 7200.10, 7200RPM hard drive
I installed both machines from the same Sandisk Cruzer USB USB flash drives. I installed Windows 7 64bit Ultimate and WIndows 8 Preview Release 64bit.
Install from power press to create your user account: 16:05
disk space used: 11,484,815,360 bytes (according to windows explorer)
To time the boot up, I disabled a password for my user account, and I created a wordpad file that would auto launch from Windows Startup. 38 seconds
Shutdown time was until machine clicked off. 11.0 seconds
Install from power press to create your user account: 26:50
Note: it got to Getting Devices Ready and Getting System ready in around 14 minutes, but hung for another 12 minutes and rebooted before I could create my user account.
disk space used: 12,580,995,072
To time the bootup, I disabled a password for my user account, and I created a wordpad file that would auto launch from Windows startup. Since METRO fires first, I would immediately click on Desktop when Metro showed and then time until my wordpad file opened. Boot up time: 29 seconds
Shut down time: Since Metro doesn't provide it's own shutdown, but rather I have to log off first and then shut down, its' a couple of clicks with my mouse. Shut down time: 15 second.
Note: Windows 8 makes use of the hiberfile to create that super fast startup. I always disable hibernation of my machines as they have SSD's and I don't want to use up disk space.
Boot time of Windows 8 after (powercfg -h off), went to 47 seconds
So, there you have it.